Add option to issue warnings instead of error in check goal

[whut]

When the check goal is used, the build fails when the source code does not adheres to google-java-format. There should be an option to instead just issue a warning and do not fail the build.

For example lets name it warningsOnly:

<build>
    <plugins>
        <plugin>
            <groupId>com.coveo</groupId>
            <artifactId>fmt-maven-plugin</artifactId>
            <version>2.9</version>
            <configuration>
                <warningOnly>true</warningOnly>
            </configuration>
            <executions>
                <execution>
                    <goals>
                        <goal>check</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

[klaraward]

Solving in #153

[klaraward]

Now live in version 2.19